The committee reviews mergers and acquisitions that could threaten national security. Its members -- from 12 federal agencies, including the departments of State, Defense, Commerce and Homeland Security, as well as the attorney general and the U.S. trade representative -- can force companies to change their deals or abandon them outright.

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They work in secret, reviewing confidential corporate data the public never sees. They can ask the president, who has final say over their work, to spike any deal they don't like.

Chevron Corp., Unocal's other suitor, doesn't have to face the committee because the prospect of one California oil company buying another hasn't raised any security fears in Washington.

Fu Chengyu, the Chinese firm's chief executive, insists his company's bid doesn't pose a threat. But on Friday, he said he is ready for the committee's scrutiny.

"We are fully prepared to participate in a (foreign investment committee) review of the transaction, and we have ... made assurances to Unocal to address concerns relating to energy security," he said in a statement.

Some in Congress see the bid as an attempt by the Chinese government to seize American oil because China National is 70 percent government-owned. On Friday, 41 members of Congress asked the foreign investment committee's chairman, Treasury Secretary John Snow, for an immediate review if the bid moves forward.

"It will become increasingly difficult for U.S.-based companies to compete for scarce energy resources on the world market against China's state- owned and/or controlled energy companies," they wrote in a letter to Snow.

A review is virtually assured. Federal law requires the committee to investigate any time a company owned or controlled by a foreign government tries to buy an American firm.

Typically, a review begins once two companies have agreed on a purchase. That hasn't happened here, although Unocal said Thursday that it would promptly begin talks with the Chinese oil company. Unocal's board agreed to Chevron's $16.4 billion takeover bid in April but received a waiver from Chevron to talk with the Chinese firm after it announced its offer Wednesday.

The standards by which the committee judges deals are somewhat vague on purpose. Although committee members examine how a deal affects national security, the law that governs their work doesn't define the term.

"There is no settled definition of national security, and that's completely intentional," said John Reynolds, a partner at the Wiley, Rein & Fielding law firm who handles cases before the committee.

"From the government's point of view, it has to be that elastic because every situation will be different, and the government would never want its hands tied."

The committee has 30 days to decide whether a deal needs thorough scrutiny or can pass as it is. Those that demand more attention are then investigated for no more than 45 days.

Most deals reviewed by the committee don't need a full investigation. Since 1988, only 25 of the committee's 1,549 cases were given a full, formal investigation. Companies try to avoid a full investigation and often modify their deals to prevent one.

"In the vast majority of cases, if you see resistance, you don't want to be one of those cases that's investigated," Reynolds said. "You pull it, you fix it and you resubmit it."

Some deals wither under an investigation's glare. In 2003, Hong Kong's Hutchison Whampoa dropped its effort to buy Global Crossing in part because of a committee investigation. Companies that persist through an investigation without winning over the committee face a final decision from the president. Such cases are rare -- just 13 since 1988.

No case has ever been sent to President Bush's desk. His father, however, remains the only president ever to block a deal investigated by the committee, rejecting a bid by the China National Aero-Technology Import and Export Corp. to buy a Seattle company making aircraft parts.